In recent years, it is proposed that a silver paste containing fine silver particles is used as a bonding material to be arranged between articles to be heated for a predetermined period of time while applying pressure between the articles, to sinter silver in the bonding material to bond the articles to each other (see, e.g., Japanese Patent Laid-Open No. 2011-80147).
In order to bond articles to each other by substituting such a bonding material for a lead solder, it is desired that the articles can be bonded (pressureless-bonded) to each other without applying pressure between the articles, as such a case that the articles are bonded to each other by a lead solder. Also in order to prevent silver oxide from being formed in the bonded portion between the articles to deteriorate the bonding strength between the articles, it is desired that the articles can be bonded to each other even in an inert atmosphere, such as a nitrogen atmosphere.
As a bonding material capable of bonding articles to each other in an inert atmosphere even if any pressure is applied between the articles, there is proposed a bonding material wherein a flux component, such as diglycolic acid, is added to a silver paste containing fine silver particles (see, e.g., Japanese Patent Laid-Open No. 2011-240406).
Moreover, if voids exist on the bonded surface when articles are bonded to each other by means of a bonding material, even if the articles are bonded to each other at a desired bonding strength, when a repeated heat cycle (e.g., a heat cycle repeated at −55 to 150° C. 500 times or more) is applied thereto, stress is applied to the voids to cracks in the bonding layer from the voids to deteriorate the reliability of bonding. For that reason, in order to satisfactorily bond the articles to each other by means of the bonding material, it is required to restrain voids from being produced in the bonded portion (to suppress a proportion (void fraction) of area, which is occupied by voids with respect to the area of the bonded surface, to less than 10%), and it is required to suppress the amount of a solvent (a dispersion medium for fine silver particles) causing voids (to increase the content of silver). On the other hand, in order to satisfactorily apply the bonding material by the screen printing using a metal mask, it is required to decrease the viscosity of the bonding material. However, the content of silver in the bonding material is decreased if the viscosity of the bonding material is decreased, and the viscosity of the bonding material is increased if the content of silver in the bonding material is increased, so that the relationship between the viscosity of the bonding material and the content of silver in the bonding material is trade-off.
In general, in order to decrease the viscosity of a bonding material of a silver paste containing fine silver particles, it is known to add a dispersant to the bonding material. As a bonding material which has a high content of silver (a small amount of a solvent to be added) and the viscosity of which can be decreased to a viscosity suitable for printing, there is proposed a bonding material wherein a phosphoric acid dispersant, such as a phosphate ester dispersant, is added to a silver paste containing fine silver particles (see, e.g., Japanese Patent Laid-Open No. 2013-4309).
However, if articles are bonded to each other by means of a bonding material containing a phosphoric acid dispersant, such as a phosphate ester dispersant, it is easy to produce voids in the bonded portion.